This invention relates to physiological electrical signal monitors and more particularly to a self-prepping multiple electrode array to connect to such monitors.
Surgical procedures are becoming more non-invasive, and as a result the use of non-invasive electrophysiological monitoring to evaluate global changes of a patient's condition during surgical procedures has increased significantly. For example, EEG monitors are now being used for monitoring cerebral function during intra-operative procedures. Of particular interest are the assessment of the effects, of anesthetics, the evaluation of asymmetric activity between the left and right hemispheres of the brain in order to detect cerebral ischemia, and the detection of burst suppression.
One of the greatest impediments to making intra-operative EEG monitoring more widely practiced in the medical community is the traditional use of multiple electrodes in the standard International (10-20) Electrode Placement on the head, primarily in the scalp. Applying them takes considerable time and expertise, requires multiple, separate and time consuming skin preparation steps, and leaves the patient's scalp and hair in disarray.
Various headsets and caps are studded with different style electrodes to speed this process, but such headsets and caps are generally not disposable (and therefore must be cleaned), need to be adjusted to accommodate the widely varying dimensions of the patients' heads, and require a considerable up-front cost. Other problems are encountered in the present medical environment when such headsets and caps are designed to be single-use disposable devices because such devices are on occasion re-used despite warnings, which results in the spread of infection. Such headsets and caps have also been used with equipment for which it was not designed, which may be a well intentioned cost saving practice, but which could result in degraded performance of the device.
The most widely used electrodes are the reusable "gold cup" style electrodes that are small, bare tin, silver, or gold plated metal cups on the end of unshielded wires that may be several feet long. Such electrodes may require that the multiple scalp and forehead electrode sites first be located by measuring and marking the head. Such sites must then be prepared before applying the electrode in order to get good electrical contact. This preparation is usually accomplished by abrading the electrode sites with a grit-impregnated solution or with some other abrasive means to remove the outer layers of skin which cause the poor electrical contact. The electrodes, up to 19 on the scalp for the full International (10-20) electrode placement, are then individually applied with adhesive to the prepared sites in contact with a blood-enriched skin layer, and are then injected with conductive electrolyte cream through the hole in the top of the electrode, thereby providing a relatively low electrical contact impedance. This process leaves the patient with abraded spots, adhesive, and electrolyte cream throughout the scalp. Frequently, contact between the metal electrode and the skin occurs, causing a time-varying offset voltage that results in "baseline wander." The electrodes also need to be placed with reasonable accuracy to achieve the standard placements or montages and to be able to repeat the same measurement at a later time.
The need to use multiple, separate preparation steps makes the set-up a very time consuming process, taking perhaps up to half an hour of a medical technician's time for even a small subset of the full International (10-20) Electrode Placement. The amount of expertise and time required to prepare a patient is presently an impediment to intraoperative EEG monitoring being more widely practiced. Also, care is needed to bundle the unshielded leads to reduce electrical noise interference. Additionally, after the procedure is over, the gold cup electrodes and any placement harness need to be cleaned and sterilized since they are not intended to be disposable.
A number of prior art multiple electrode assemblies have been developed for EEG monitoring. U.S. Pat. No. 4,595,013 issued to Jones; U.S. Pat. No. 4,928,696 issued to Henderson; U.S. Pat. No. 4,638,807 issued to Ryder; U.S. Pat. No. 4,072,145 issued to Silva; and U.S. Pat. No. 3,490,439 issued to Rolston are several examples. These multiple electrode assemblies, however, all require some or all of the multiple, separate and time consuming steps of skin preparation described above to reduce the contact impedance with the skin before they are applied to the body. These separate skin preparation steps also make it difficult to improve contact impedance once the electrode has been applied to the patient or after the medical procedure is underway. If the preparation was inadequate at the time the multiple electrode assembly is applied, it must be removed, the skin reabraded, and most likely a new electrode assembly would have to be reapplied, adding additional expense to the additional preparation time. Too much abrasion can cause a skin injury, or bleeding, leaving the patient with a lasting wound. Separate devices required to abrade the skin cause the risk to the applicator by potential contact with blood and by possible disease transmittal during preparation.
There are also a number of prior art multiple electrode assemblies that are self prepping. U.S. Pat. No. 4,709,702 and associated electrode U.S. Pat. No. 4,640,290, both issued to Sherwin, utilize an array of spring loaded metal "tulip" electrodes in a reusable headset that penetrates the outer dead layers of skin to achieve a low contact impedance. Also, U.S. Pat. No. 4,770,180 and associated electrode U.S. Pat. No. 4,706,679 both issued to Schmidt utilize an array of stiff, bundled metal wires that contact and penetrate the patient's skin. The drawback with both of these assemblies is that the metal contact with the skin causes highly undesirable time-varying offset voltages that interfere with the sensitive measurement of the small signal voltages of the body. Also, both of these assemblies, and other assemblies that utilize a headset or cap such as the assembly described in U.S. Pat. No. 4,967,038 issued to Gevins, need some adjustment to properly position the electrodes on the widely varying dimensions of the patients' heads, and require a high up-front cost and cleaning after use.
U.S. Pat. No. 4,936,306 issued to Doty utilizes a spiral coil electrode that may be metallic, and that uses cork-screws into patient's skin to achieve low contact impedance. While this may achieve low contact impedance, it has the significant drawbacks of discomfort to the patient and creating sites of possible infection because of the deep skin punctures made by the spiral coils. If made of metal, the spiral coils will also cause time-varying voltages. Lastly, these electrodes are actually applied individually since they must be screwed into the patient's scalp, which adds time to the procedure.
U.S. Pat. No. 4,683,892 issued to Johansson utilizes a headset with multiple electrodes that are activated by compressed air, which impinge against the patient's scalp, and that also dispense electrolyte paste to improve contact. This is a complex and expensive device, not intended for general, routine use in an intraoperative environment.
It is therefore a principal object of the present invention to provide a disposable, pre-gelled, self-prepping multiple electrode array which easily and reliably prepares the skin to assume a relatively low contact impedance.
Another object of the present invention is to provide a self-prepping multiple electrode array that does not require the use of more than one component to be handled by the person applying the device, and fits most head sizes in the general patient population.
Still another object of the present invention is to provide a multiple electrode array that can monitor cerebral function without the use of electrodes placed in the scalp, and that is easily aligned on the head.
A further object of the present invention is to provide a multiple electrode array that prevents its use with monitoring equipment with which it was not intended to be used.